The present invention relates to a method and apparatus for obtaining negative and positive images and, more particularly, to a technique utilizing an image-forming apparatus for forming color-separated original images necessary for forming a press form for offset printing.
Along with recent developments in electronics, photoengraving techniques have advanced remarkably, and electronic photoengraving is the leading trend. In the electronic photoengraving, the gray scale levels of an original image are photoelectrically converted to an electrical signal, and the electrical signal is subjected to predetermined processing to be converted to light of variable intensities. A film (photosensitive material) is exposed to light of variable intensities, and is developed to form a negative or positive film serving as a press form.
However, this method requires very complicated processes, and a machine therefor becomes bulky, resulting in very high cost. Conventionally, the machine is used only for forming press forms, and cannot be used in other applications.
Another method is proposed wherein films (block copies) for forming press forms for each of a plurality of colors are formed by a transfer type color copying machine.
With this method, however, it is difficult to check colors (i.e., in the finished image) when a press form is formed in practice by the films (block copies) and printing is performed using this press form.
For example, in a thermal transfer type color copying machine, a thermal transfer ink ribbon having yellow, magenta, cyan, and black portions is used to form color-separated original images on films. Thus, actual print colors can be known only after the print colors of all the films (in this case, four films) for press forms for the respective colors are mixed. The same problem is posed if the film to be formed is either a negative or positive film.
The thermal transfer ink ribbon is normally disposed after being used for forming a negative or positive film. However, its disposal poses many important problems.
First, the disposed ribbon may be misused by a third party unless its disposal is carefully managed. Since the thermal transfer ink ribbon used for formation of a negative or positive film has film formation information, i.e., press form information, highly confidential information may be disclosed to a malicious third party.
Second, it is unpreferable from the viewpoint of effective use of resources that the used thermal transfer ink ribbon is simply disposed.
When both the negative and positive films must be formed for a single original, the amount of the thermal transfer ink ribbon used is twice that in the case wherein either one film is formed, thus causing a still further unpreferable result.